While hallucinogenic drugs of abuse are believed to generate their psychotropic effects through their interaction with 5-HT2/5-HT1C subtype receptors, the hallmark of this interaction which is specific to their unique neuropsychological actions is unknown. The recent isolation of multiple cDNA clones representing the 5-HT2, 5-HT1C and other high affinity hallucinogen binding sites allows the characteristics and basis of hallucinogen-receptor interactions to be probed at the molecular level. Two related but distinct hypotheses are consistent with present experimental data. The parameters critical to the psychotropic effects of 5-HT ligands might involve differences in their binding site, activation or coupling specificity. Alternatively, the psychotropic effects of hallucinogens may derive, in part, from their interaction with one of the newly described receptors which bind hallucinogens with high affinity. To examine these hypotheses, we propose to integrate molecular biological, computational molecular modeling (Weinstein component of this Interactive Research Project Grant-IRPG) and pharmacological approaches (Maayani component) to probe the basis of hallucinogen action. The specific residues involved in the binding of hallucinogens to the 5-HT2 and 5-HT1C receptors will be explored. The residues selected for mutation will be based upon residues known to be critical for other GPCRs, empirical considerations and hypotheses derived from molecular modeling. The structural basis for the differences in ligand binding and receptor activation/effector coupling between the 5-HT2 and 5-HT1C receptors will be investigated by coordinated mutation of each. The spatial and functional relationships of the transmembrane helices will be probed to validate the ongoing modeling (Weinstein component) and thereby to enable more precise hypotheses about the sites of hallucinogen interaction to be developed and tested. Additional 5-HT subtypes are being identified rapidly via molecular cloning and our studies will be extended to other human receptors which emerge as particularly compelling targets of hallucinogens. The coordinated study of receptor mutants with psychotropic and non psychotropic ligands will help clarify the molecular basis of the actions of these chemicals and may suggest novel therapeutic approaches to modulate the effect of these drugs of abuse.